Optical fibre cables for transmitting high optical power are frequently used in industrial applications. Specifically they are used in cutting and welding operations by means of high-power laser radiation, but also in other industrial applications such as heating, detection or working operations in high-temperature environments this type of optical fibre cables can be used. By means of the optical fibres it is possible to build up flexible manufacturing systems for transmitting radiation from the high power laser source to the workpiece. Normally, an optical fibre has a core of glass and a surrounding cladding. Laser sources which can be used in this context have average power from a few hundred watts up to several kilowatts.
When designing fibre systems for such high power laser radiation it is important to take care of radiation that falls outside the core of the fibre, due to for instance reflections against the workpiece or due to an incorrect focussing to the fibre, and cool it down in order to prevent uncontrolled heating in the system. Different methods to take care of such undesired power radiation are already known. One example is disclosed in DE 4305313, in which the radiation that falls into the cladding of the fibre is spread in a so-called mode stripper and absorbed by a metal surface. This surface can then be cooled from the outside of the device. A similar method is described in EP 0 151 909.
An optical fibre in which at least one of the end surfaces of the fibre core is provided with a rod having a larger diameter than the core diameter is described in EP 0 619 508. At this end the fibre is provided with a reflector designed to conduct rays entering outside the fibre towards an area where they can be absorbed without causing any damage. In the illustrated embodiment this area is surrounded by a heat-abducting device with cooling fins, but it is also mentioned that water cooling means may be included in this area for cooling off the generated heat. Also in this case the cooling is provided from the outside of the device. A similar arrangement in which the end part of the fiber is provided with a hollow rod and a reflector is described in GB 2 255 199.
In SE 509 706 and RU 2031420 it is described a method to take care of power loss in which the radiation heat completely or partially is absorbed directly in a flowing coolant instead of a metal. In SE 509 706 at least one of the contact ends of the fibre is located in a cavity filled with a flowing coolant so that radiation falling outside the fibre is entered into and absorbed at least partially by the coolant. According to a preferred embodiment the fibre is directly in contact with the surrounding coolant, for example water. The advantage by having the radiation absorbed directly in the coolant is a more effective cooling as no heat conducting through for instance a metal part is required before the heat is cooled off.
Even if the methods which have been described so far provides an effective cooling they do not indicate the level of the real power loss. Reflections on the workpiece always means that some part of the power returns to the fibre contact and generates a heating. In order to control the process it is of interest to measure the power loss in the fibre connector.
Under the development of lasers the diameter of the optical fibers has been decreased. A normal fibre diameter in the middle of the ninetees was around 0.5 mm, while recent lasers have fibre diameters down to approximately 0.1 mm. Then there are increasing demands on a correct focussing in the optical system. This has mostly been satisfied by means of active methods. It is then necessary to find out good methods for measuring the optimal position of the optical fibre.